Conventional lean burn compression ignition engines such as diesel engines produce diesel particulate matter (PM) emissions including soot and volatile and soluble organic fractions. Future legislated limits for PM require the installation of a particulate filter in the exhaust system.
A known problem with soot filters is that they are plugged with soot during operation, which requires continuous or periodical regeneration to prevent excessive pressure drop over the filter. Periodical regeneration is conventionally performed by increasing the temperature in the exhaust system to a temperature, where the captured soot is oxidized to gaseous components and thus removed from the filter.
Typically, the required temperature increase is obtained by injecting extra fuel, which is burned over a diesel oxidation catalyst (DOC) placed in the exhaust system upstream the filter. A DOC is already installed in many diesel vehicles to meet the emissions limits for hydrocarbons and CO. Both the increased pressure drop over the filter and the periodical regeneration gives rise to a fuel penalty compared to an engine exhaust gas system without a particulate filter.
Different systems have been developed in order to lower this fuel penalty. One such system is disclosed in U.S. Pat. No. 4,902,487, which has been commercialized as the “Continuously Regenerating Trap”. In this system NO2 is used for combusting diesel particulate deposited on a filter at lower temperature than possible with oxygen also available in the gas. NO2 is obtained by oxidising NO being present in the exhaust gas over a suitable catalyst placed upstream of the filter. This NO oxidation catalyst typically contains Pt, which is known as an excellent NO oxidation catalyst. To further increase the NO2 content in the exhaust gas, filters have also been coated with a Pt containing catalyst. As a disadvantage of such systems, NO2 can slip past the filter and be undesirably exhausted to atmosphere, when there is insufficient PM on the filter to react with NO2 generated over the oxidation catalyst or the temperature of the exhaust gas is below a preferred range for combustion of PM in NO2.
DE102005027063A1 discloses a device for after treatment of oxygen containing exhaust gases of internal combustion engine has nitrogen dioxide reduction catalytic converter which is provided as part at flow end of exhaust gas after treatment device being provided with an oxygenation catalyst and/or a soot filter. The SCR catalyst for the reduction of nitrogen dioxide contains is impregnated with small amounts of platinum metals.
WO06040533A1 relates to decomposition of nitrogen dioxide to nitrogen monoxide in lean-burn internal combustion engine by contacting acidic metal oxide with exhaust gas. The acidic metal oxide is selected from the group consisting of zeolites, tungsten-doped titania, silica-titania, zirconia-titania, gamma-alumina, amorphous silica-alumina and mixtures thereof.
NO2 is toxic in low doses. It is apprehensive that the levels of NO2 in European cities and highways increase [See presentations from “NO2 Workshop, Munich February 2006”]. Legislative authorities have therefore begun to discuss limiting the amount of NO2 to an acceptable level being permissible in the exhaust gas. For example the yearly average downtown Stuttgart was in 2004 68 μg/m3, which by order shall be reduced to a limit of 40 μg/m3 NO2 (yearly average) in Stuttgart in 2010. Accordingly, it is presumed that reduction of both PM and NO2 in exhaust gas emissions to the atmosphere will be required by law.
It is thus a general object of the present invention to provide a method and apparatus for the substantial reduction of NO2 and preferably NO2 and PM in the exhaust gas from a compression ignition engine.